The present invention relates to a piston-cylinder unit suitable for use in, for example, a tailgate of a vehicle to open the tailgate upon operation of the unit through a partial extension stroke of the piston, stop the opening at the end of the partial stroke, and upon application of a manual force to the tailgate open the tailgate beyond the partial stroke.
DE 3,301,544 A1 discloses a piston-cylinder unit in the structural form of a gas spring, which is used for opening gates capable of swinging upward, for example, the tailgates of vehicles. The special feature of the gas spring lies in that two opening angle regions are predetermined. In a first opening angle region, the gas spring, because of the compressive force acting on the cross section of the piston rod, extends automatically until the piston has reached the end of a bypass groove that connects the two working chambers which are separated by the piston.
In the region of the second opening angle region, fluid exchange is influenced by a pressure-control valve that consists of a helical spring and a valve seal. In use, the pressure-control valve can be opened by an additional external force, the manual force of the user, whereby the gate can be moved into the second opening angle region.
As soon as the gate is released, the valve seal, biased by the spring, opens a valve aperture. The gas cushion confined in the working chamber remote from the piston rod holds the gate in the desired position in the second opening angle region. If the gate is to be moved further, manual force is again required.
A basic problem, which is common to all gas springs, is that the extension force is highly temperature-dependent. There are many possible solutions to this problem. One possibility, for example, is to maintain the temperature of the gas in the gas spring at a desired value by electric heating. DE 4,421,773 A1 describes such a system. A fundamental disadvantage is that the expenditure for equipment and input of energy are of course considerable.
U.S. Pat. No. 5,404,972 describes a self-blocking gas spring with a temperature-dependent bypass valve. The temperature-dependent bypass valve contains a bimetallic spring whose elastic force increases with decreasing temperature and, in so doing, prestresses a valve in the direction of closing. Depending upon the temperature, the valve is opened progressively by the operating pressure in the working chamber on the piston-rod side in order to keep the push-out force of the gas spring as constant as possible.